The present invention relates to a liquid crystal display apparatus capable of effecting high-speed display with a high contrast.
Almost all the commercially available active matrix-type displays utilize a TN (twisted nematic)-mode using a nematic liquid crystal.
However, a liquid crystal display device using such a TN-mode is accompanied with difficulties such as a low response speed (e.g., a large response time of several ten msec to several hundred msec) and a poor viewing-angle characteristic, thus resulting in inferior image qualities.
In order to overcome these difficulties, high-speed liquid crystal devices have been studied and proposed. As a representative example thereof, liquid crystal devices using a ferroelectric liquid crystal or an anti-ferroelectric liquid crystal have been developed. By using these liquid crystal devices, it is possible to realize a high-speed responsiveness (several ten psec to several hundred xcexcsec) and a wider viewing angle. These liquid crystal devices are proposed, e.g., by T. Yoshida et al., xe2x80x9cA full-color thresholdless Antiferroelectric LCD exhibiting wide viewing angle with fast response timexe2x80x9d, SID (Society for Information Display) 97 DIGEST, p.841- or T. Saishu et al., xe2x80x9cVoltage-holding properties of thresholdless antiferroelectric liquid crystals driven by active matricesxe2x80x9d, SID 96 DIGEST, p.703-.
A liquid crystal material used in these articles is a smectic liquid crystal showing thresholdless antiferroelectricity and used in combination with a pair of cross-nicol polarizers for optical modulation. One optical axis (an absorption axis or transmission axis) of (two optical axes of) the pair of polarizers is aligned with an (average) optical axis of (anti-ferroelectric or smectic) liquid crystal molecules placed in a state where no electric field is applied, i.e., an antiferroelectric alignment state. These optical axes are also aligned with a direction normal (perpendicular) to a smectic (molecular) layer extension direction (hereinbelow, referred to as a xe2x80x9clayer normal directionxe2x80x9d).
When a liquid crystal device and the pair of cross-nicol polarizers are arranged in such a state, a positive-polarity electric field and negative-polarity electric field having an identical magnitude (as an absolute value) provide an identical optical state (response characteristic) when these electric fields are applied to the liquid crystal device. Accordingly, the above-described arrangement of the liquid crystal device and the polarizers is convenient for drive of the liquid crystal device with an AC driving waveform.
A transmittance T (%) of a liquid crystal device is represented by the following formula:
T(%)xe2x88x9d sin2(2xcex8),
wherein xcex8 denotes an angle formed between an optical axis of (two optical axes) of a pair of polarizers and an (average) optical axis of a liquid crystal.
As apparent from the above formula, in order to provide a maximum transmittance, a condition of xcex8=45 degrees is required but the antiferroelectric liquid crystal material generally provides xcex8=30-35 deg. as also described in the above-mentioned articles. As a result, it is difficult to obtain the maximum transmittance in the above-described case where one of two optical axes is aligned with the layer normal direction of smectic liquid crystal molecules, thus resulting in a low contrast.
Incidentally, in order to develop a liquid crystal display device allowing smooth motion picture display, in addition to an increase in response speed of the liquid crystal material per se, studies from the viewpoint of human engineering have been promoted (e.g., K. Sueoka et al., xe2x80x9cImproving the moving-image quality of TFT LCDsxe2x80x9d, International display research conference (IDRC) 1997, p. 203-).
According to this article, a CRT (cathode ray tube)-type display device is an impulsive light-emission display device for each frame period. On the other hand, a TFT (thin-film transistor)-type liquid crystal device is a light-quantity holding display device for holding a light quantity during each frame period. In view of smoothness of motion picture display, the former impulsive display allows smooth motion picture display due to response characteristics including an after light characteristic on human eyes but the latter light-quantity holding type display device is accompanied with less visibility of motion picture image, such as a double image phenomenon (a phenomenon such that the image is seen double by a viewer) since the afterlight in the preceding frame period still remains in human eyes in the current frame period. In order to realize the smooth display of motion picture image in the TFT-type liquid crystal device, one frame period is divided into two periods including a half (50%) display period and a half non-display period.
As described hereinabove, in the conventional liquid crystal display devices, a high-speed liquid crystal material such as the antiferroelectric liquid crystal is suitable for the TFT-type liquid crystal device allowing the motion picture display but provides a smaller tilt angle, thus failing to realize a sufficient transmittance. In order to obtain smooth motion picture image display in the TFT-type liquid crystal device, it is necessary to effect display with a timewise opening rate of about at most 50%.
In view of the above-mentioned problems of the conventional liquid crystal display devices, an object of the present invention is to provide a liquid crystal display apparatus providing not only a sufficient transmittance (transmitted light quantity) by using a liquid crystal (such as an anti-ferroelectric liquid crystal) showing a high-speed responsiveness and a wide viewing angle but providing a relatively small saturated tilt angle (below 45 deg.), but also a timewise opening rate of at most 50% for meeting motion picture display and a sufficient continuous image forming performance.
Another object of the present invention is to provide a liquid crystal display apparatus capable of providing a high-speed display with a high contrast.
According to the present invention, there is provided a liquid crystal display apparatus, comprising:
a liquid crystal device comprising a pair of substrates each having thereon an electrode and a liquid crystal disposed between the pair of substrates and providing a tilt angle,
driving voltage application means for applying a driving voltage to the liquid crystal, and
at least one polarizer having a polarizing axis, wherein
said liquid crystal has a voltage-dependent alignment characteristic such that the tilt angle continuously changes depending on the driving voltage applied to the liquid crystal, and
said at least one polarizer is disposed so that the polarizing axis is shifted from positions providing a maximum transmittance and minimum transmittance under application of no electric field.
According to the present invention, there is also provided a liquid crystal display apparatus, comprising:
a liquid crystal device comprising a pair of substrates each provided with an electrode and a liquid crystal disposed between the pair of substrates to form a plurality of pixels arranged in a matrix form,
drive means for driving the liquid crystal device, and
a pair of cross nicol polarizers disposed to sandwich the liquid crystal device, wherein
the liquid crystal is an antiferroelectric liquid crystal which provides a thresholdless voltage-transmittance curve inherently assuming a V-character shape having a symmetry axis across a zero-voltage point if one of optical axes of the polarizers were aligned in a direction normal to molecular layers of the antiferroelectric liquid crystal,
but said one of optical axes of the polarizers is actually set to form a prescribed angle with said direction normal to molecular layers thereby to provide an asymmetrical voltage-transmittance curve with respect to a line across a zero-voltage point, and
said drive means comprises means for driving the liquid crystal device in alternately repeated display and non-display periods to compensate for a DC voltage component.
Herein, the term xe2x80x9ctilt anglexe2x80x9d refers to to an angle formed between an average molecular axis (optical axis) of the liquid crystal and the layer normal direction of the liquid crystal molecules.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.